This invention relates to light spot scanning, and more particularly to scanning with the use of holographic optical elements.
The primary function of a scanning system is the controlled sampling, or restoration, of information. In an optical scanning system, the information is processed either in parallel by a light beam which can simultaneously illuminate many data sites, or sequentially by a beam which, due to its size, illuminates only a single data site at a time. Interest in sequential optical scanning has expanded in recent years primarily because of new capabilities provided by laser light. Laser scanners are capable of generating high resolution images at high scan rates. Most of the scanning systems devised to manipulate a laser beam include a galvanometer, rotating mirror, acousto-optic elements, or electro-optic elements as the light deflector. It was first demonstrated in 1967 that a rotating hologram can also serve as a deflector element in an image scanning system.
Laser line scanners used for imaging applications are generally required to generate a repetitive single scan line. A problem which has been encountered with multi-faceted rotating mirror line scanners is that due to the facet-to-facet nonuniformities and spinner wobble, non-collinear multiple scan lines are formed. An obvious solution to this problem is to fabricate the spinner assembly to such precise mechanical and optical tolerances that the residual error does not detract from the desired level of image quality. The expense of this approach, however, is a decided disadvantage. Holographic scanning provides an alternative by which this problem can be minimized.
In a typical arrangement for making a flat holographic spinner, a point light source serves as the object and a normally incident plane light wave as the reference beam. When a hologram thus constructed is illuminated with a plane light wave which is the conjugate of the original reference beam, the hologram functions to reconstruct a wavefront which converges with or without the use of additional optical elements, to form an image of the original point object light source. When the holographic spinner is then rotated about the axis of the reference beam, the reconstructed image spot scans a circle in space.
A problem exists in that, if the spinner is rigidly mounted to the rotating shaft but is not perpendicular to the shaft, mechanical wobble of the spinner creates a distorted reconstruction image and/or mechanical breakdown of the spinner. The elastomeric mounting technique disclosed in U.S. Pat. No. 4,067,639 provides significant improvement but causes the spinner to asymptotically approach the perpendicular position without fully reaching it and, hence, a small degree of wobble still exists.
It is an object of this invention to provide a holographic spinner which is free from mechanical wobble during rotation.
Another object is to provide a holographic spinner which freely rotates into a fully perpendicular position with respect to the axis of rotation.
Other objects, advantages and features of this invention may become apparent from the following more detailed description given in connection with the accompanying drawings.